In this paper, the transversal strain on fiber Bragg grating (FBG) is specially analyzed according to the couple-mode theory. For both cases, plane strain and plane stress, numerical simulation is done for uniform FBG. In the case of plane strain, one peak of the reflection spectrum shifts towards the longer wavelength, while the other shifts slightly towards the shorter wavelength and in the case of plane stress, both peaks shift towards the longer wavelength. Finally, a plan of experiment on FBG under lateral load is designed to examine the conclusions. Good agreements between experimental results and numerical simulations have been obtained. The results of the experiment indicate that the transversal strain is between the case of plane strain and plane stress and more close to the case of plane stress. Since the transversal strain and the axial strain have different contributions to the wavelength shift based on the theoretical analysis, it is possible to obtain the measurement of three-dimensional strain from the Bragg reflection wavelength variation.